Given our recent launch of 2008 US Voter Info, we thought we'd share a behind-the-scenes look at where this data comes from. Google is collecting official election information from many sources - our earlier blog post noted that we've worked with state and local election officials and partners in the Voting Information Project, including the Pew Charitable Trusts and League of Women Voters. Each has helped us get access to data necessary for answering your questions about where to vote.

The best source of data is the "feed" of election information provided directly by state and local election officials in the Voting Information Project's open format. VIP partners provide us a full set of data that matches voter addresses to polling places for their respective jurisdictions. With this data, we can do a great job finding your polling place.

In other jurisdictions we've tapped a few sources of official election information: we've collected data that matches registered voter addresses to precincts, and another set of data that matches precincts to polling places. Overall, this data is very useful but there are some challenges since both datasets could have errors and omissions.

The key problem is that we have to extrapolate which precincts voters are assigned to based on a single snapshot of a national voter database, which is a moving target. It is at this stage, in particular, that we have to be careful not to be wrong. If you live on a street not represented in this snapshot then we cannot know what your polling place would be because we can't safely match it to a precinct.

This is a conservative algorithm, but we think it's the best approach. For information this important, we would rather not provide an answer than the wrong answer. When we can't find a polling place, we provide links to state election officials' websites and other voter resources.

Overall, our current coverage of US polling places is 92%. This means that even if we can match an address to a precinct, 8% of the time we won't know what the polling place is for that precinct -- but the vast majority of people should be able to find exactly what they need to know. Of course, we're still working on getting as close to 100% as possible, so keep checking back with us!